This invention relates to an improved process for applying inorganic conductive coatings on substrates, and is particularly concerned with procedure for modifying the physical properties, particularly the viscosity, of precursor solutions to facilitate application of metal sulfide, e.g. nickel sulfide, conductive coatings or conductive patterns on substrates.
As disclosed in U.S. Pat. Nos. 5,002,824 and 5,041,306, both to Warren, electrically conductive inorganic coatings can be applied to a substrate such as fiberglass fabric by contacting the substrate, as by dipping or spraying, with a precursor solution of a metal salt, such as nickel sulfate, and a sulfur donor such as thiourea. The resulting treated substrate is then dried and heated to form an electrically conductive metal sulfide, e.g. nickel sulfide, adherent coating or pattern on the substrate, while preserving the physical properties.
The addition of other ingredients to the precursor solution can adjust the conductivity and improve the mechanical properties, e.g. shelf-life stability, of the deposited conductive film. Selective patterning of such conductive films or coatings can be achieved by various printing processes, and also such conductive coatings have application on components for controlling electromagnetic fields, such as aircraft edge surfaces, e.g. the edges of wings.
The above noted precursor solution when applied to a substrate, evaporates prior to reaction of the components therein to form the conductive metal sulfide. Particularly when employing spraying as the means for applying the precursor solution to a substrate for producing conductive patterns, it is difficult to control the evaporation rate. Control of electrical conductivity of the deposited metal sulfide requires that the mass of the precursor material which is applied to a substrate be carefully controlled. Control of the mass of the coating is often desired in order to achieve some other property than electrical conductivity, such as weight, color, depth or thickness.
An improved method for applying the precursor which will provide better control of mass transfer of precursor and of placement of electrically conductive patterns on a substrate is desirable. Conventional methods such as spraying and dipping are not able to provide the predictability in this respect that is required.
It is an object of the invention to provide an improved precursor solution of a metal salt and a sulfur donor, for production of a conductive coating on a substrate, providing better control of mass transfer of the precursor and of the placement of conductive coatings and patterns on a substrate.
Another object is to increase the viscosity of the precursor solution and control the evaporation rate of the solution, particularly when applied by spraying, to facilitate application and control of the conductive film on the substrate, particularly for the production of conductive patterns, or to function as an ink in the screen or gravure processes, or in film transfer processes.
A still further object is to control the fluid properties, including viscosity and wetting power, of the above precursor solution.
Yet another object is to provide a procedure for applying the improved precursor solution to a substrate to provide a controlled conductive coating or pattern.
Other objects and advantages of the invention will appear hereinafter.